The invention relates to a squirrel-cage rotor of an electric machine having a cage that passes through a stack axially and is made of electrically conductive rotor bars connected by short-circuit rings at the ends projecting from the stack.
In larger three-phase motors operated with frequency converters with varying rpm, e.g. in electric motor vehicles and locomotives, but also in other applications, the generally conventional construction of the rotor leads to problems. In the standard configuration as a cage rotor, the tubular stack comprises magnetic lamina layered on the rotor shaft to prevent eddy currents, wherein grooves cut into the outer diameter of the lamina receive short-circuit copper rods (i.e., rotor bars). At the end of the stack, the rotor bars project beyond the lamina and are connected there to end-side short-circuit rings by a welded or soldered connection. The rotor bars disposed in the sheet grooves and the short-circuit rings secured at the end form the cage. For electrical-related reasons, the short-circuit rods generally have a greater expansion in the radial direction than in width. The rods projecting beyond the stack thus have less flexural strength in the tangential direction of the rotor than in the radial direction. The short-circuit ring connected with material-to-material bonding to the rods is oscillatory in the tangential direction with respect to the stack because of the softness of the short-circuit rods. When considered alone, the rotor can be viewed as a rotational oscillation system comprising three masses. The stack forms the central, large mass on both sides of which the short-circuit rings are connected as further rotational masses by means of reinforcements against torsion formed by the short-circuit rods.
In larger three-phase motors, the natural torsional frequencies that result from this oscillation system are less than 1000 Hz. If this type of machine is operated with frequency converters, the occurring harmonic waves lead to notable pendulum torques in the current. These pendulum torques act on the rotor as an outside torque and can excite it, again as an oscillatory system, to torsional oscillations. When this occurs the short-circuit rings oscillate in torsional fashion against the stack. Under unfavorable conditions, resonances result that cause unacceptably high mechanical tension on the rods, which are stressed to the point of bending, and possibly damage to the motor.
These fundamental connections ensue from, for example, the reference "ETZ-Archive", Vol. 6 (1984), issue 5, pages 165 to 172.
To realize higher natural torsional frequencies on the rotor and prevent resonance effects in that the natural rotary frequencies are higher than the excitation frequencies from the frequency converter, the connection between the stack and the short-circuit rings can, of course, be configured to be more resistant to torsion. For example, the projection of the rods beyond the stack can be shortened so that the bending length of the rods is shorter. This option is, however, limited, because a certain rod projection is necessary due to the mechanical stress on the rods at high rotary rpm in the field of centrifugal force.